1. Field of the Invention
The invention is directed in general to electric arc furnaces, and more particularly to a new power source and its control for electric arc furnaces, such as three-phase electric arc furnaces, which enables the reactive power consumption to be kept constant, thus reducing flicker at the point of common coupling. Furthermore, the dynamic performance can be increased. Such arc furnaces may be used, for example, for the production of steel and iron from scrap-metal, for melting non-ferrous materials such as for example aluminum and in chemical processing industries. Of course, such furnaces are not limited to use in the foregoing manners, and instead are contemplated for use in a variety of settings.
2. Background Art
Generally, for an electric arc furnace, such as a three-phase electric arc furnace, the commonly used power source consists mainly of a furnace transformer, which is generally connected directly to a high voltage power system. In particular, the prior art such a connection in principally two different manners. First, this may be accomplished by way of a block circuit comprising pre-transformers and a furnace transformer or through the most approved variation by the direct connection of the furnace transformer with an intermediate circuit to the power system. This connection operates with medium voltage and is therefore relatively less expensive.
In the second case, the furnace transformer with the intermediate circuit comprises a main transformer and a magnetic-decoupled booster transformer (See, for example, Kaempf, P.; Markworth, E.; Mxc3xchlenbeck, J.: 110 kV-Lichtbogen-schmelzofen mit Lastschaltung im Zwischenkreis, Stahl u.Eisen 94(1974) Nr.9, S. 393 and Brehler, R.: Ofentransformatoren zum Speisen von Lichtbogenxc3x6fen mit Ofenschalter im Zwischenkreis, Siemens-Zeitschrift 50(1976), Nr.1, S. 9-17).
In such an example, the booster transformer has a half-rated power relative to overall furnace power. The voltage of the intermediate circuit is set at a medium voltage level (NEC Standard). The furnace switch operates with a medium voltage (NEC Standard) and can be switch on with a lower furnace power (50% of the furnace power). In such an example, a dynamic Var compensator meant for compensating fundamental and distortion reactive power can be integrated into the intermediate circuit. Lastly, the furnace is connected to the high voltage power system through the high voltage circuit breaker.
In another such prior art embodiment, a further modification of the above power source has been made. Such a modification comprises a thyristor current controller and a variable inductance per phase (See, for example, Dicks, U.S. Pat. No. 4,677,643). The thyristor current controller with the variable inductance short-circuits the main reactance of the booster transformer whenever the main thyristor current controller faces a current cut off and is blocked.
Generally, with all of these conventional three-phase electric arc furnaces, undesired distortions in the power system having a low short-circuit capacity in the form of voltage fluctuations occur. This results in a flicker, at the point of common coupling. Such flicker is due to the fluctuating reactive power consumption of the furnace. In addition, with respect to the embodiment shown in Dicks ""643, the inductance produces power loss and the circuit is associated with a low dynamic response due to the longer dead time produced between the changes in the delay angle.
To solve the foregoing problems, sophisticated and expensive electrical equipment, such as a dynamic Var compensator, is required to compensate for the distortions. Further, other disadvantages to such systems are the poor quality of power regulation, as the power yield can be varied only in stages and only in greater time distance. As a result, this process can lead to wear of the switching components.
Thus, it would be an object of the invention to provide an electronic-controlled furnace transformer together with a new control concept, which enables the reactive power consumed by the furnace to be kept constant. This is made possible if the conventional power system of the three-phase electric arc furnace disclosed in the ""643 patent is supplemented with a zero voltage switch instead of the variable inductance. The new control concept enables the regulation of the (reactive) power input and through this method the disturbances in the power system and the excessive power loss can be reduced to a minimum. Moreover, in one embodiment the dynamic Var compensator can be replaced with a less complex and relatively cheap static Var compensator (SVC).
The invention comprises a power source for an arc furnace having an intermediate circuit transformer with a AC main-driven converter. The AC main-driven converter comprises a current controller and a zero voltage switch. Both of the current controller and the zero voltage switch are integrated in the intermediate circuit.
In a preferred embodiment of the invention, the arc furnace comprises a three-phase arc furnace and the a AC main-driven converter comprises a three phase current controller and zero voltage switch.
In another preferred embodiment, the invention further includes means for regulating the reactive power through the phase control of the three-phase current controller.
In another preferred embodiment, the invention further includes means for compensating for unsymmetrical loads by regulating the phase control of the three-phase current controller in each phase separately.
In yet another preferred embodiment, the converter in the intermediate circuit includes means for maintaining the power system current substantially constant.
In yet another preferred embodiment, the intermediate circuit includes a reactive power compensator that comprises a static or dynamic Var compensator.
The invention further includes a method for controlling the output voltage of a furnace comprising the steps of: (a) rough adjusting output voltage with a transformer; and (b) fine tuning the output voltage by varying the firing angle of the converter.